An ink-jet head of an ink-jet printer is provided with an insulated substrate on which pressure chambers are provided and a nozzle plate in which nozzles are provided. Ink is supplied to the pressure chamber, and the nozzle discharges an ink droplet. The pressure chambers are formed by providing a plurality of grooves on a piezoelectric element member on the insulated substrate, and each has an electrode at a side surface and a bottom surface thereof. When a voltage is applied to the electrode, the piezoelectric element deforms and thereby the volume of the pressure chamber varies. The pressure chamber is reduced to cause the ink inside the pressure chamber to be pressurized, and thereby the ink is discharged from the nozzle as an ink droplet.
In order to prevent that a current flows from the electrode to ink, an organic protection film is usually formed on the surface of the electrode contacting ink. Though having high electrical insulation properties, the organic protection film also has high water-shedding property and is easy to shed water. In order to prevent air bubbles from being involved in the ink, it has been proposed that a hydrophilization treatment such as a UV treatment and a plasma treatment is preformed on the organic protection film to thereby improve the hydrophilic property thereof. When an inorganic film with high hydrophilic property is formed on the organic protection film, the hydrophilic property of the surface contacting ink can be improved, and a sputtering method or a PE-CVD (Plasma-Enhanced Chemical Vapor Deposition) method is used for forming an inorganic film.
In addition, it has been proposed that at the time of forming a hydrophobic insulating film having water-shedding property opposite of hydrophilic property on an electrode, an atomic layer deposition (Atomic Layer Deposition: ALD) method is used.
The effect of hydrophilization by a plasma treatment which is performed on an organic protection film goes down as time passes, and may dissolve in several weeks. In a manufacturing process of an ink-jet head, heating and ultrasonic cleaning are sometimes performed after a plasma treatment. In this case, the effect by the plasma treatment is reduced to thereby cause the air bubbles to be involved when water-based ink is used.
With a film forming method such as a sputtering method and a PE-CVD method, it is difficult to form an inorganic film with a uniform thickness in a whole fine pressure chamber. The film is formed thick at the upper position in the pressure chamber, but it is difficult to form the film at the lower portion, and thereby the hydrophilic properties become different by location. In addition, at the time of forming an inorganic film with such a method, the temperature within a film forming tank becomes to a high temperature of not less than 150° C. The organic protection film is damaged with such a high temperature, and thereby the effect to protect the electrode from the ink drops.